1. Field of the Invention
The purpose of the method of the invention is to transfer heat from a hot fluid (A) to a cold fluid (B) and more particularly to recover available heat from a hot fluid for transferring it to a cold fluid to be heated.
2. Description of the Prior Art
In numerous cases, such a heat exchange cannot be effected under satisfactory conditions by placing the hot fluid and the cold fluid directly in exchange relation in a heat exchanger. This is particularly the case when these two fluids flow in ducts spaced apart from each other, the fact of bringing them into contact in an exchanger leading to a cumbersome and costly installation or else to unacceptable pressure losses. This may occur more particularly for exchanges between gases flowing at relatively low pressures.
In such a case it is known to use a heat carrying agent such as water, water containing glycol or else liquid organic fluids with a high boiling temperature, flowing in an exchange loop. The heat carrying fluid being heated by the hot fluid in a first heat exchange zone and heating the cold fluid in a second heat exchange zone separate from the first one.
Such a system requires the permanent operation of a circulation pump which involves maintenance for ensuring reliable operation over a long period of time. Furthermore, none of the fluids used is entirely satisfactory. Water used without antifreeze cannot be used in winter in most cases of application; water containing glycol which overcomes this disadvantage has characteristics of high viscosity adversely affecting the heat transfer and leads to corrosion risks. Finally, heavy organic fluids are expensive and also have a high viscosity.
It is further known that heat transfer may be accomplished by vaporization and condensation of a fluid such as water or an organic fluid; however, such a technique is not adapted to the heat exchange between fluids whose temperature varies during the exchange and in particular cannot be used if the temperature ranges for the hot fluid and the cold fluid partially overlap.
A heat transfer system using a heat carrying fluid flowing in a circuit forming a loop has been described by Guiffre et al. in the U.S. Pat. No. 4,314,601.
This system comprises an evaporator, a condenser and a central collector connected together by a loop circuit (FIG. 2 of Guiffre et al.). In this system the fluid leaving the evaporator is mixed in the central collector with the fluid leaving the condenser, which means that the temperature of the fluid leaving the evaporator is lowered whereas the temperature of the fluid leaving the condenser is increased, thus the inlet temperatures of the evaporator and of the condenser are respectively higher and lower than those at the outlet of the condenser and of the evaporator. The increase in the enthalpy of the fluid between its leaving the condenser and its entry into the evaporator results in limited efficiency of cooling of the external fluid; similarly, reduction of the enthalpy of the fluid entering the condenser results in an overall relatively limited efficiency of heating of the external fluid. The overall efficiency of the heat transfer of this system between hot fluid and cold fluid is therefore relatively low. Furthermore, the use of the system, coupled to the use of fluid mixtures, leads to obtaining different concentrations of each fluid in the condenser and in the evaporator which corresponds to different temperature ranges: it will therefore in such a case be difficult to work with partial overlap of the ranges of variation of the temperatures of the hot fluid and of the cold fluid.
The U.S. Pat. No. 4,216,903 describes a heat exchange system comprising an exchange loop using as heat carrying fluid, for example, a halogenated hydrocarbon or a mixture of halogenated hydrocarbons. Heat exchange with an external fluid in the condenser, for heating water, takes place in the aggregate in counter current fashion, whereas the heat exchange at the condenser, for reheating air, takes place in aggregate in cross current fashion and the heat exchange with an external fluid in the evaporator takes place in the aggregate in co-current fashion. The system comprises a liquid reserve of heat carrying fluid situated between the outlet of the condenser and the inlet of the evaporator and at least a U shaped tube whose topmost part is situated at a level between the lowest level of the evaporator and the highest level of the evaporator, which defines the flow direction of the heat carrying fluid.
The use of non azeotropic mixtures, such for example as those described in the patent application EP No. 57,120, in the above described system, means that the system cannot correctly respond to a variation of the input temperature of the external fluids and/or to a variation of the flow rate of these fluids.